Conventionally, there is a camshaft driving mechanism comprising a crank sprocket, a cam sprocket and an idler sprocket disposed therebetween. A first timing chain spans the crank sprocket and the idler sprocket while a second chain spans the idler sprocket and the cam sprocket.
The number of revolution of the camshaft must be one half of that of the crankshaft. Therefore, a speed reduction is effected between the second timing chain and the first timing chain in the above-mentioned camshaft driving mechanism. Thus, the second timing chain is used at a speed slower than that of the first timing chain, but its load capacity is higher.
However, in the conventional driving mechanism, chains of the same type (size) are used for both the first and second timing chains. This means that the first chain is excessive in terms of the load capacity.
On the other hand, since the first timing chain runs at a speed greater than that of the second timing chain, being large-sized itself can be a cause of noises. Moreover, the two timing chains being of the same size the driving mechanism is susceptible to resonance. In addition, a larger mounting space is required adjacent to an engine itself.